1. Field of the Invention
The present invention relates to an electric discharge machining apparatus that machines a work piece by means of an electric discharge generated by application of a voltage between a tool electrode and the work piece.
2. Description of the Related Art
A conventional electric discharge machining apparatus has hitherto been known in which a tool electrode is moved in an X-axis, a Y-axis and a Z-axis direction by using an electrode driving device with a magnetic bearing mechanism being applied to driving the tool electrode for electric discharge machining (for example, see a first patent document: International Publication No. 02/024389 A1).
In this conventional electric discharge machining apparatus, an electric motor by which the tool electrode is driven to rotate is incorporated in the electrode driving device.
In the electric discharge machining apparatus, however, the tool electrode is fixed to an end of a drive shaft that drives the tool electrode, and the drive shaft also serves as a rotor of the electric motor. As a result, the gross weight of the drive shaft is increased, so high-speed or fast responsive drivability of the tool electrode might sometimes be impaired, resulting in a problem that the rapidity or fastness of the machining speed is reduced.
In addition, the fast responsive drivability of the tool electrode can be obtained by increasing the attractions of electromagnetic portions of the magnetic bearing, that is, the value of current supplied to the electromagnetic portions. However, the amount of heat generated in the electromagnetic portions increases in accordance with the increasing current supplied, as a consequence of which the electrode driving device might sometimes be caused to thermally expand depending upon the amount of heat thus generated, giving rise to another problem that machining accuracy is reduced. 